1. Field of the Invention
The present invention relates to a substrate for an electronic device, an electronic device, and methods of manufacturing such a substrate and an electronic device that are applied to semiconductor devices such as light-emitting diodes and laser diodes, dielectric devices such as thin-film capacitors and ferroelectric nonvolatile memories, and piezoelectric devices such as thin-film bulk acoustic oscillators and surface acoustic wave elements.
2. Description of the Related Art
A variety of electronic devices have been designed and made practical. Such electronic devices are made of various types of substrates on which functional films made of, for example, semiconductors, dielectrics or piezoelectric materials are formed. Examples of electronic devices using semiconductor films are light-emitting diodes and semiconductor lasers. Examples of electronic devices using dielectric films are ferroelectric nonvolatile memories and infrared sensors. Examples of electronic devices using piezoelectric films are thin-film bulk acoustic oscillators.
For such electronic devices the functional films made of, for example, semiconductors, dielectrics or piezoelectric materials are important part that determines the performance characteristics of the electronic devices. Accordingly, it is required to form the functional films that exhibit an excellent crystallinity to obtain the electronic devices having enhanced performance characteristics. To form the functional films that exhibit an excellent crystallinity, it is important to not only optimize the method and conditions for film formation but also optimize the crystallinity of the base layer on which each functional film is formed. The crystallinity of the base layer is particularly important when the functional film is formed through epitaxial growth.
The functional films of many of electronic devices are each formed on a conductor film that serves as a lower electrode. In this case, the conductor film as the lower electrode preferably has an excellent crystallinity.
The Published Unexamined Japanese Patent Application Heisei 11-260835 (1999) discloses a substrate for an electronic device formed by stacking a buffer layer, a surface layer and a metal thin film one by one on a single-crystal Si substrate, wherein the surface layer includes a nitride epitaxial film having a wurtzite crystal structure and/or an oxide epitaxial film having a wurtzite crystal structure.
The Published Unexamined Japanese Patent Application Heisei 11-312801 (1999) discloses a multilayer thin film having a metal thin film that is an epitaxial film of (100) single-oriented cubic crystal, and a buffer layer having a {111} facet located at the interface with the metal thin film.
According to the techniques disclosed in the above-mentioned Published Unexamined Japanese Patent Application Heisei 11-260835 and Published Unexamined Japanese Patent Application Heisei 11-312801, the buffer layer is formed on the substrate through epitaxial growth, and furthermore, the conductor film to be the lower electrode is formed on the buffer layer through epitaxial growth. The conductor film having an excellent crystallinity is thereby formed.
According to the techniques disclosed in the above-mentioned Published Unexamined Japanese Patent Application Heisei 11-260835 and Published Unexamined Japanese Patent Application Heisei 11-312801, the buffer layer and the conductor film are formed on the substrate through epitaxial growth. As a result, there arises a problem that it is difficult to fabricate the conductor film having an excellent crystallinity because, for example, a process performed at a temperature of 900° C. or higher is required.
When an epitaxial film is intended to be formed directly on the substrate, the following problem may arise. The epitaxial growth means that crystal growth is effected in such a manner that the atoms are oriented in accordance with the orientation of the atoms of the crystal of the base layer. Therefore, if the base layer is made of an amorphous material such as glass or silicon oxide, it is impossible to form an epitaxial film on such a base layer.
It is possible to easily obtain a Pt thin film having an excellent crystallinity in which the (111) plane is single-oriented to be parallel to the substrate surface through a method in which a film of silicon oxide or silicon nitride, for example, is formed on a silicon (Si) substrate, and on this film, a platinum (Pt) thin film as a conductor film is vacuum-deposited at a temperature around 500° C. However, the Pt thin film has a serious problem that an absolute contact is not established between the Pt thin film and the base layer and the Pt thin film is easily removed, which affects the reliability of the process and the electronic device.
If a Pt film is formed directly on a glass substrate, the contact between the glass substrate and the Pt film is not sufficient. If a metal thin film is formed directly on a Si substrate, silicide is likely to be formed and it is difficult to obtain a metal thin film having a good crystallinity.
The inventors of the present invention have considered a method of forming a metal thin film on a substrate, an adhesion film of Ti, Cr, TiO2, or Al2O3, for example, being disposed between the substrate and the metal thin film. Through this method, however, it is impossible to form a metal thin film having a crystallinity sufficient for forming a functional film having a good crystallinity on the metal thin film.
As thus described, it is difficult through the conventional methods to form a conductor thin film on the substrate, the conductor thin film having both of an excellent crystallinity and an excellent adhesion property that allows a close contact with the substrate. Novel techniques for achieving this object have been sought.